The investigation on acoustic pressure oscillation effect on the mean burning rate of solid propellant combustion under quasi and unsteady conditions has been carried out at elevated pressures typical of rocket operation. Experiments were conducted in the window bomb test facility to measure mean burning rate for the mean chamber pressure ranging from 1 to 7 MPa. This analysis facilitates current understanding regarding the mean burning rate variation due to the acoustic wave interaction with propellant flame complexes. The present work is focused on low frequency combustion instability and characterization of the long rocket motor configurations. The window bomb is connected with an exclusively designed rotating valve for the generation of acoustic pressure oscillations at selected mean chamber pressures and the corresponding frequency of operations. Two kinds of ammonium perchlorate/hydroxyl terminated poly butadiene (AP/HTPB) based composite propellants, one with aluminium, and the other without aluminium were considered. Experiments were conducted at the excited frequency of 140 Hz with different excited acoustic pressure amplitudes. This experimental setup facility has the provision of visual observation during propellant combustion using optical methods. The performance of the rotating valve was competent over the tested conditions. These studies reveal that the acoustic pressure amplitudes significantly enhance the mean burning rate of the solid propellants. The response of the aluminized propellants to the excited acoustic oscillations was relatively prominent as compared to that of its non-aluminized counterparts. The results obtained are extremely useful in combustion instability characteristics of large scale rocket motors.